This invention relates to Pitot tube type devices, and especially those for actuating marine speedometers.
Heretofore, various forms of Pitot tube devices have been proposed for actuating marine speedometers. These devices generally include a streamlined body member secured to the transom of the boat. An impact pressure-transmitting tube extends through the body member from a sensing orifice and is connected to the speedometer. The impact pressure tube transmits to the speedometer, or to a transducer which drives the speedometer, the impact pressure or velocity head which results from the relative movement of the boat with respect to the water.
This basic type of Pitot tube apparatus, in one or another form is used with practically all pleasure craft, and in general operates satisfactorily. However, if during the operation of such craft, the orifice of the impact tube should become clogged with marine weed growth or floating debris, simple forms of this type of device fail to function. In an attempt to alleviate some of the problems which result from use of the water craft in areas containing weed growth or floating debris, the hollow blade or body member has been positioned so as to be pulled through the water at an extreme rake angle of approximately 45.degree.. Further, the orifice or stagnation point opening of the impact tube has been shrouded behind the leading edge of the blade for additional protection. Also, many prior designs incorporate some form of spring-loaded mounting bracket that allows the blade section to be pivoted into a substantially horizontal position which also facilitates beaching or trailer loading of the water craft. This spring-loaded mounting bracket also allows the blade to move up and over small submerged objects at low speeds. An example of one such Pitot tube apparatus may be found in commonly owned U.S. Pat. No. 3,181,356 to Carpenter, entitled Pitot Tube and issued on May 4, 1965.
Pitot tube constructions of the type just noted, although generally providing adequate performance at relatively low speeds, have suffered certain deficiences as boat speeds have increased due to the availability of larger engines and jet-drive devices. These deficiencies have generally been related to the use of the shrouded impact orifice as well as the spring-loaded mounting bracket construction.
For example, in order to hold the blade in the proper position in the water at a 50 knot speed, the mounting bracket spring must be of such strength so as to provide almost no protection as a yieldable pivot device at cruising speeds. In effect, the high spring rate required results in a substantially rigid or fixed mounting of the Pitot tube blade for low speeds. Contact with underwater objects may thus result in severe damage to the blade. Also, prior Pitot tube designs which transmit the impact pressure or velocity head directly to the marine speedometer have resulted in fluctuation of the speedometer reading, particularly at high speeds and on rough water. Any transient pressure conditions due to momentary changes in the rake angle of the blade have been transmitted directly to the speedometer, resulting in surging or fluctuation of the speedometer readout.
One known attempt to maintain the Pitot tube in substantially the same position regardless of boat speed may be found in U.S. Pat. No. 3,084,543 to Finkl, entitled Pitot tube and issued on Apr. 9, 1963. The device disclosed therein includes a blade or body member pivotally connected to the transom of a boat. A paravane positioned adjacent the bottom edge of the body member is employed to counteract the force of the water which tends to rotate the Pitot tube out of position. Although compensating for increases in boat speed, this device is still prone to blockage of the impact orifice by marine growth and/or debris.
Therefore, a need exists for a Pitot tube type apparatus adapted for an improved marine speedometer wherein the position of the Pitot tube body or blade is maintained substantially constant regardless of boat speed and yet may be pivoted towards a horizontal position upon impact with underwater objects which readily sheds marine growth and/or debris capable of blocking the impact tube orifice, and, which may be pivoted to a horizontal position for beaching or transport of the boat on a trailer.